The invention relates to an X-ray fluorescence analysis device and to a method for carrying out such analyses and the use of this device. In particular, the device is appropriate to the trace element analysis, and comprises possible application fields, particularly both in the environmental analysis and thin film technology (semiconductor technology).
The highest detection sensitivity with the X-ray fluorescence analysis one achieves in that X-radiation having an angle of incidence at which total reflection occurs is directed upon a sample on a sample carrier wherein the angle of incidence has to be lower than the critical angle xcex8c of the total reflection which is caused by the matter properties of the sample carrier material. The critical angle xcex8c results from the refractive index (n=1xe2x88x92xcex4+ixcex2; 1xe2x88x92xcex4xe2x86x92real part [dispersion], ixcex2xe2x86x92imaginary part [absorption] of the sample carrier material with xcex8c={square root over (2xcex4)} (xcex8c [rad] as an radian measure). The greater is the photon energy of the used radiation the lower is the critical angle xcex8c, and being approximately 0.1xc2x0 with e.g. Mo K type irradiation for a quartz carrier. However, the lower is the angle of incidence xcex8c the greater is the projection of the primary beam section upon the sample carrier. Therefore, the sensitivity of detection (related to the number of atoms to be detected per surface) of the total reflection X-ray fluorescence analysis (TXRF) is essentially limited by the obtained photon flux density of the exciting primary radiation per surface unit.
Increasing the photon flux density of the X-radiation by means of focussing polychromatic or monochromatic radiation is possible but does not merely have any practical significance due to the small angles xcex8c.
With the total reflection the radiation portion refracted within the sample carrier is degraded toward the surface wave exponentially fading into the depth, and the penetrating depth (several nanometers only, int. al.  less than 10 nm) of the X-radiation into the sample is controlled in a dispersive manner. Typically, more than 95% of the incident radiation will be reflected.
By the limitation of the sample carrier volume excited with the sample to a few nanometers a very high signal-to-background ratio is to be written down. The portions of the background generated in this area in the vicinity of the surface are reaching the detector in an unattenuated form, however, and will be consequently measured therewith.
A measuring arrangement based on this findings is described in EP 0 456 897 A1. There, an X-ray source is used wherein the X-radiation thereof is directed upon a sample carrier by means of at least one reflecting unit under the conditions of total reflection. The fluorescence radiation thus excited is measured by a detector, and the sample is respectively analysed. A second detector senses the X-radiation reflected on the sample carrier, and the control of the critical angle of the total reflection is allowed to be readjusted by means of an angle resolved type measurement with respective handling the reflector unit and the sample carrier.
The respective sample is immediately placed on the sample carrier. A periodically repeating sequence of one or more layers made of materials having a different refractive index is formed on the reflector unit. On that occasion, layers are mentioned wherein having such a pair of layers, at least one layer is relatively large-atomic. Thus, so called multi-layer mirrors are used to diffract the X-radiation on the reflector unit. However, the fluorescence excitation exclusively occurs under the conditions of the total reflection on the sample carrier in acceptance of the mentioned and well known drawbacks.
Therefore, it is an object of the invention to increase the detection sensitivity of most different samples.
This object preferably is achieved by the characterizing features of the present invention. Advantageous embodiments and further developments of the solution will be apparent from the description of the invention provided herein.
With the device according to the invention the radiation from an X-ray source for the fluorescence excitation of a sample which can be solid or fluidic is directed upon a multi-layer system serving as a sample carrier. The multi-layer system comprises a number of at least two layers which are made of materials having a different x-ray optical refractive index, respectively. The period thickness d of one period meets the BRAGG relationship in consideration of the incident angle xcex8in of radiation with xcex upon the surface of the multi-layer system. The period thickness d and the sequence of individual layers is allowed to be constant for the respective adjacent periods in the multi-layer system wherein in this case it is allowed to be said of periodically built-up multi-layer systems.
In addition, there is also the way to modify the structure of the multi-layer system by changing one or several individual layer thicknesses such that in contrast to the periodical structure a higher reflectivity and/or a lower angular acceptance is achieved with the predetermined xcex8m and wavelength xcex of the incident X-radiation photons. In this case it is being said of a periodically constructed multi-layer systems.
With the use of one or several reflector unit(s) it has to be taken account of the period thickness dj of each surface element Aj of the multi-layer system on a reflector unit which has to be selected such that the X-radiation photons reflected from this multi-layer system and directed upon the multi-layer system arranged on the sample carrier meet the BRAGG relationship in the sample location. On that occasion, influencing the angle of incidence of the radiation can take place by moving the sample carrier and/or with at least one reflector unit which is arranged in the optical path between the X-ray source and the surface of the multi-layer system.
The radiation is allowed to be directed upon the used multi-layer system on the sample carrier in consideration of the BRAGG relationship
m*xcex=2deff*sin xcex8m
wherein
m is a whole number 1, 2, 3 . . .       d    eff    =      d    ⁢          (              1        -                              δ            _                                              sin              2                        ⁢                          θ              m                                          )      
xe2x80x83wherein
d is a period thickness of the multi-layer system, and       δ    _    =            1      d        ⁢                  ∑                  i          =          1                m            ⁢              xe2x80x83            ⁢                        d          i                ⁢                  δ          i                    
is a mean value weighed according to the thickness of the real parts of the refractive index of all layers of one period, with an angle of incidence being substantially greater than it is the case with the total reflection. On that occasion, the photon flux density/surface element of the exciting X-radiation can be greater by the factor sin xcex8m/sin xcex8c as it is the case with the total reflection as the reflected radiation portion is in the same procentual range as with the total reflection. Herein, xcex8m is the BRAGG angle of the mth maximum which is greater than the critical angle xcex8 of total reflection, and which is substantially determined by the period thickness d of the layer pairs of the multi-layer system, and which is not determined by the xcex4-values of the layers.
The highest reflectivities of multi-layer systems can be found within the first order (m=1).
Depending on the amount of reflectivity a continuous wave field is forming on the surface of the multi-layer system arranged on the sample carrier which can be employed for the fluorescence excitation of the sample. With an appropriate choice of the period number, period thickness and layer materials in the multi-layer system reflectivities can be achieved which almost are corresponding to those with total reflection(R greater than 90%).
The mulitlayer system is allowed to be formed on a substrate, e.g., with well known methods of thin-film technology (plasma enhanced chemical vapour deposition PLD, chemical vapour deposition technique CVD, sputtering or other). The individual layers and the substrate should be composed of elements and compounds of elements having the atomic numbers of Z less than 10, respectively. The fluorescent lines of the layer materials and substrate materials, as is the case, thereby are not located within the power interval of the elements to be detected in the sample, and they do not contribute to the background. Thereby, elements having greater atomic numbers can be detected with a higher detection sensitivity.
Appropriate elements include H, B, Be, Li, N and C and the compounds B4C and CNx as well. Favourably, the various modifications of carbon such as diamond-like or graphite-like carbon can also be used in a pair of layers. Organic compounds consisting of C, N, O and H are conceivable as well.
Glassy carbon is an appropriate substrate material whereon a respective multi-layer system can be formed. Glassy carbon is commercially availabe, e.g. with the indication of xe2x80x9cSigradurxe2x80x9d.
It is also advantageous to use such a multi-layer system for beam forming and/or for the monochromatization on at least one reflector unit thus the fluorescent lines of the reflector unit(s) are not located within the energy interval to be detected.
In contrast to the conventional total reflection X-ray analysis, it is possible with the invention to increase the photon flux density of the exciting X-radiation several times without deteriorating the signal-to-background ratio. In addition, more effecient focussing of polychromatic X-radiation is allowed wherein in the following it will be noted yet to possible implementations therewith. With focussing it should be considered that each photon meets the BRAGG condition in the impact point upon the multi-layer system. This can be implemented if the wavelength of the incident photons increases according to the BRAGG relationship with the angle of incidence thus occurring a direction dependent monochromatization of each partial beam.
Complying this condition, with a reflector unit it is allowed to focus upon the surface of the multi-layer system arranged on the substrate carrier a polychromatic exciting radiation having an aperture angle of the convergent radiation which is several times greater than the angle of incidence for total reflection.
By means of an elliptically curved gradient layer system upon at least one reflector unit a divergent polychromatic X-radiation is thus allowed to be converted into such a polychromatic, i.e. direction dependent monochromatic radiation which can be focussed according to the reflection conditions upon the surface of the multi-layer system arranged on the sample carrier.
In an analogous manner parallel polychromatic radiation can be focussed upon a sample by means of parabolically curved gradient multi-layer systems.
For focussing the X-radiation upon a mulitlayer system arranged on the sample carrier one mulitlayer system of a reflector unit having the form of a equiangular spiral can also be used.
If the radiation is not focussed upon the multi-layer system arranged on the sample carrier thus the BRAGG relationship means that monochromatic parallel radiation has to be incident.
By means of a respective parabolically curved multi-layer system which period thicknesses d of the layer pairs thereof are selected as gradients of period thickness, a divergent polychromatic X-radiation is allowed to be converted into a monochromatic parallel radiation wherein the period thickness depression in the multi-layer system on a reflector unit ensures that the X-radiation photons impacting upon the multi-layer system arranged on a sample carrier meet the BRAGG relationship.
Parallel polychromatic radiation can be converted into a monochromatic parallel radiation by means of a plane multi-layer system having a constant period thickness d.
For simultaneous proofing the fluorescence radiation of different elements an energy dispersive detector can be used for the detection of fluorescence.
Advantageously, the detector is to be cooled for energy dissolving wherein a Peltier cooled detector having a Lithium Peltier element or liquid nitrogen can be used.
In addition, it is possible to place poly-capillary optics between the detector and multi-layer system arranged on the sample carrier to enable a locally resolved type measurement. On that occasion, the focus of the poly-capillary optics is set toward the sample by means of an elevation adjustment.
For different applications it is favourable to moderate the sample carrier. Thus, solid and liquid samples, respectively, are allowed to be vaporized by warming, and the fluorescence analysis can be carried out thereafter, if required.
However, for particular samples and analyses, respectively, cooling can make sense as well.
Synchrotrons, rotating anodes or X-ray tubes as well can be used as the X-ray sources. On that occasion, merely the various radiation divergences (e.g. micro focus type tubes or fine focus type tubes), and wavelengths in particular with the layout of the multi-layer system(s) are to be taken into account. The modular structure of the measuring arrangement ensures the simple adaptation to the various X-ray sources.
It is also advantageous to monitor complying the BRAGG angle and the BRAGG maximum by means of a second detector. This detector measures the intensity of reflected radiation of the multi-layer system arranged on the surface of the sample carrier. On that occasion, sensibly a locally resolved type measurement occurs, e.g. with the line shaped detector or with a detector array. According to the radiation sensitivity measured the beam guidance and the incident angle of the exciting radiation upon the surface of the multi-layer system can correspondingly be varied here such that the maximum intensity will be measured. Herewith, the position of the multi-layer system on the sample carrier with respect to the incident X-radiation can be adapted, e.g. by horizontal and/or vertical shifting or by pivoting about a horizontal axis in order to measure within the optical range.
However, a respective influence can also be achieved solely or in addition to respective motions on the reflector unit(s). In each case, however, a positioning way is desirable in at least one degree of freedom for the sample carrier and/or the reflector unit.
If several reflector units are subsequently arranged in the optical path of the exciting radiation, the beam direction is allowed to be influenced in a most different manner according to the number and angular orientation.
This invention can be advantageously used with environmental analysis, and here, in particular, with proofing the minimum concentrations of toxic and other matters, respectively.
Another application would be the online type inspection of vacuum deposition methods wherein the deposition atmosphere can be monitored.
In the semiconductor technology/microelectronics the demands relating the chemical purity of wafers continuously increase. Consequently, the detection sensitivity has to be enhanced as well. This is enabled by combining the well known VPD method with the invention and lowering the detection limit which is thus achievable.
As already mentioned samples of most different phase can be analysed.
(a) However, with respect to fluids it is significant to guide them in accordance with specific flow conditions into and over the analyzing area, respectively, on the surface of the multi-layer system arranged on the sample carrier.
(b) Thus, particle shaped samples are allowed to be analysed on a multi-layer system.
(c) A liquid sample can be gained, e.g. by means of VPD (Vapour phase deposition) from a Si wafer, or such as e.g. sea water it can be extracted directly from the environment.
The device comprising its various elements can be formed in a closed housing. In the housing the respective analysis can be carried out in a vaccum. However, detecting is also possible within an inert nitrogen atmosphere or helium atmosphere or other gas atmospheres (Z less than 10) if the fluorescence, e.g., of argon contained in the air largely contributes to the background.
In the following the invention shall be explained by way of example.